1. Field of the Invention
This invention relates to novel composite pigment products comprising from 4.5% up to 50%, by weight, of particulate titanium dioxide.
More specifically, this invention relates to composite pigment products in which particles of titanium dioxide and other pigmentary and subpigmentary components, as well as disperse and soluble nonpigmentary components, are coflocculated and cemented intrinsically with the aid of in-situ synthesized complex (multicomponent) functional microgels.
2. Discussion of the Relevant Art
White pigments encompass a class of particulate materials which are essentially colorless, insoluble, nontoxic, reasonably nonabrasive, and have dimensions favoring a diffuse reflection, or scattering, of light constituting the visible portion of the electromagnetic spectrum with wavelengths ranging from 420 nm for violet to 660 nm for red.
In accordance with the laws of physical optics, maximum scattering of light occurs when a propagating light wave encounters in its path an obstacle, a pigment particle as the case in point, whose dimensions are equal to one-half of the length of the impinging wave. At equal particle dimensions, pigmentary materials with higher refractive indexes, whose values may range from 1.41 for silica to 2.73 for rutile, scatter the light more efficiently than analogous materials with lower refractive indexes.
The most elementary physical model of light scattering may be considered one in which monochromatic light is diffracted by a single spherical particle. Maximum diffraction of the blue, green and red portions of the light spectrum (additive primary components of light) is obtained with particle diameters of about 150 nm, 200 nm and 250 nm, respectively. By integrating the comprehensive spectral response of a single spherical particle scattering polychromatic light, mapped as a function of particle diameter, one can calculate that the maximum light scattering occurs with a particle of about 200 nm in diameter.
A universally useful model of light scattering by pigments must, however, be applicable both to any arbitrary pigment shape (virtually all inorganic pigments, other than TiO.sub.2, are nonspherical) as well as to integral end-use formations containing these pigments, such as paper-coating and paint films, filled paper or pigmented plastics. Let us consider, for example, a single, highly anisometric particle of kaolin clay in the form of a hexagonal platelet. The light waves of different lengths impinging upon such a multifaceted platelet scatter with different intensities depending on how closely the dimensions of a particular facet of a clay platelet approximate one-half of the length of the impinging light wave. Among the multitude of facets by which the impinging light may be scattered are, for example, platelet faces (in the x,y plane) or edges and protrusions from platelet's surfaces (in z direction). Moreover, the impinging light waves are scattered independently by each of the six triangular tips of the hexagonal platelet, the shorter waves being scattered more efficiently closer to the tips, across shorter distances, and, correspondingly, the longer waves being scattered more efficiently farther from the tips, across longer distances.
The ability to scatter light is a universal property both of particulate and extensive matter. Hence, even an "infinitely" large, most precisely polished mirror also scatters light, though only to a very negligible extent. In general, all light waves, regardless of length, will scatter with different intensities across all physical obstacles encountered in their path, such as individual particles or parts of aggregated matter, grain boundaries, or sites of localized stress concentrations giving rise to elastooptical effects.
As far as white pigments are concerned, it should be emphasized that the latter represent a pragmatic class of particulate materials, useful in the trade, whose features are defined by a convention. In the very minimum, pigments must consist to a predominant extent of particles whose dimensions uniquely favor the scattering of light, not so much with regard to the performance of individual particles but primarily with regard to the performance of the resultant end-use formations containing these particles. The latter requirement necessitates that pigments additionally possess certain specific features and performance properties, though the scope of the latter is not fixed but expands steadily in keeping pace with the scientific and technological advancements in the field of pigments.
Whether a solid particle can be classified as pigmentary depends not only upon raw physical dimensions but also upon the particle's morphology. Hence, monolithic, spherical, virtually perfectly isometric, single-faceted particles of TiO.sub.2, or organic pigments, cease to be pigmentary for all practical purposes when their particle diameters exceed about 1 .mu.m. Overall, a spherical shape for mineral pigment particles is disadvantageous in many respects. Spheres, being perfectly isometric, single-faceted geometric bodies, scatter the light more selectively, hence, less efficiently, than analogous anisometric particles of an equivalent mass. Moreover, spheres have an inherent tendency to form dense, closely packed structures with a low void volume, characterized by a low light-scattering efficacy. Closely packed ensembles of monodisperse populations of spheres have a maximum void volume of only about 26%, which, for polydisperse populations of spheres, can fall below 15%, or even 10%.
The formation of closely packed ensembles of pigment particles, the closest such packing occurring with spherical ones, is unavoidable in paper coating and filling or in paints, causing many potential light-scattering sites to become inaccessible to the impinging light waves. Hence, the magnitude of integral light scattering of a formation of closely packed spherical particles is invariably much less than the sum of potential light-scattering effects attainable with the individual component particles scattering the light as discrete spheres.
Multi-faceted pigment particles, such asintrinsically aggregated clusters of elementary, ultrafine (sub-pigmentary) particles of precipitated silica or metal silicates, on the other hand, can be as large as 10 .mu.m, or even 20 .mu.m, e.s.d. (equivalent spherical diameter) and still be pigment worthy. Regardless of morphological features, however, all particulate materials with dimensions finer than 0.1 .mu.m e.s.d. are not pigment worthy, being classified as "subpigmentary." It should be pointed out, though, that inherently fine-particle-size pigment products, such as titanium dioxide or high-glossing kaolin clay, may contain a substantial proportion of subpigmentary particles.
White pigments traditionally have been divided in the art into "primary," with a refractive index of about 2.0 or higher, and "secondary," with a refractive index ranging from about 1.4 to 1.65. Following the example of lithopone, introduced on the market around 1875, interspacing of particles of high-refractive-index primary pigments with particles of low-refractive-index secondary ones has become a standing practice in the paint and paper industries. As the first composite pigment ever, obtained by coprecipitating birefringent zinc sulfide (refractive indices 2.356 and 2.378), used in proportions of from 30 to 60%, by weight, with barium sulfate (refractive index 1.64), lithopone represents a classical example of a virtually perfect interspacing of a primary pigment with a secondary one. As titanium dioxide (TiO.sub.2) was introduced on the market in 1919, quickly becoming the dominant high-refractive-index white pigment on the market, it became instantly clear that the most economical performance is obtained when TiO.sub.2 is used in blends with less expensive, low-refractive-index co-pigments (extenders). It was also recognized, however, that a great deal of detrimental selective fractionation and flocculation occurs in practical applications involving the use of such loose pigment blends. Hence, various composite pigments were developed in which the primary pigments, such as titanium dioxide or zinc sulfide (ZnS), were first intimately blended with, and subsequently affixed to, secondary pigments (extenders), to achieve a permanent immobilization of both particulate species relative to each other.
The initial main approaches used in the prior art to manufacture composite pigments containing particulate titanium dioxide involved either a precipitation of the latter in a slurry of a secondary pigment (barium sulfate or calcium sulfate), or a simultaneous coprecipitation of both the titanium dioxide and secondary pigment, followed by dewatering, calcining and pulverization. The above TiO.sub.2 -based composite pigments, whose manufacture was based in part on a simulation of the lithopone process, were called "coalesced" composite pigments. The intrinsic cementation of TiO.sub.2 /extender aggregates, ensuring their mechanical integrity, was obtained by thermal sintering.
The subsequent approaches toward synthesizing analogous composite pigments were based on various methods of treatment of aqueous slurries of extraneously prepared primary and secondary pigments to obtain maximum mutual interspacing and a permanent immobilization of the primary and secondary particulate species relative to each other. Accordingly, the in-situ coprecipitated, TiO.sub.2 -based composite pigments of the prior art can be classified as "intrinsic" composite pigments, to distingush them from the "derivative" ones, synthesized from extraneous pigmentary components.
It should not be overlooked, however, that the secondary pigments (extenders, co-pigments) used along with titanium dioxide are, as a rule, highly polydisperse systems characterized by a wide spread of particle dimensions. Hence, while the use of co-pigments is, overall, beneficial to the resultant optical-performance efficacy of titanium dioxide, the oversized particles in co-pigments have a well-documented adverse effect upon the flocculation characteristic of the relatively small TiO.sub.2 particles, this adverse effect being particularly strongly pronounced in dilute and non-optimally dispersed slurries.
The effect of larger particles on the flocculation tendency of smaller particles was first described by V. D. Samygin et al. in the article titled "Mechanism of Mutual Flocculation of Particles Differing in Size" (translated from Kolloidnyi Zhurnal, Vol. 30, No. 4, pp. 581-586, July-August, 1968), dealing with the flocculation phenomena in flotation processes. According to the above article, the rate of adhesion of fine particles to coarser ones may be higher by a factor of 10.sup.3 -10.sup.4 than the rate of cohesion between finer particles. Applicant's subsequent experimental work showed that the above phenomenon is rather universal and is encountered in both dry and wet disperse systems. For example, coarser and more abrasive particle aggregates were obtained by calcining (thermal sintering) of high-glossing clay feeds with particle dimensions ranging from 0.1 to 2.0 .mu.m e.s.d. than by calcining of analogous feeds from which essentially all particles larger than about 1.5 .mu.m e.s.d. were removed by centrifugal separation.
The first of the above-mentioned novel approaches to the manufacture of derivative composite pigments, based on flocculation (coflocculation) of pigmentary components suspended in aqueous media, was disclosed by Alessandroni in U.S. Pat. Nos. 2,176,875, 2,176,876 and U.S. Pat. No. 2,176,877. In one case, for example, the coflocculation was carried out by adding an extraneous flocculant to an aqueous slurry containing both the primary, high-refractive-index pigment (titanium dioxide or zinc sulfide) and the secondary, low-refractive-index extender pigment. In another case, the coflocculation of pigmentary components was attained when a separately prepared aqueous slurry of titanium dioxide, or zinc sulfide, dispersed with one type of dispersant, was blended with a separately prepared slurry of an extender pigment dispersed with another type of dispersant "antipathetic" to the former one. In both of the above cases, the flocculated media were filtered, dewatered, dried and pulverized without the calcining step being employed.
Based on the present colloid-chemical experience and in view of the preceding discussion of the article by Samygin, however, it is clear that a high degree of detrimental separation and selective aggregation of the particulates, according to both species and size, could not have been avoided with the slow and inefficient flocculation mechanisms employed by Alessandroni. Moreover, the approaches used by Alessandroni are lacking a viable adhesion mechanism capable of imparting an adequate mechanical integrity to the resultant composite pigments.
Another approach toward the synthesis of white and colored derivative composite pigments was disclosed in U.S. Pat. No. 3,453,131 to Fadner. According to the latter, single and/or multiple species of functional colloidal particles of ". . . carbon black, acetylene black, iron oxide, Mannox blue, azobisisobutyronitrile, zinc oxide, methyl zimate, sulfur, titanium dioxide, polystyrene, or antimony oxide or mixtures thereof" with diameters ranging from 0.01 .mu.m to 1.0 .mu.m were attached by means of a "coupling agent" to platy clay particles, ranging from 0.5 .mu.m to 3.0 .infin.m in diameter, used as a carrier medium.
The above composite pigments were synthesized by adding 0.5% to 25%, by weight, of an aliphatic acid (coupling agent) into an aqueous slurry of pigmentary components and ". . . mixing the composite suspension for a sufficient time to form the composite colloidal particles." The resultant "composite particle suspensions" were considered as the final products intended for use in various commercial formulations in which the individual component materials have traditionally been employed in a loose (nonaggregated) state.
U.S. Pat. No. 3,453,131 to Fadner also teaches that, "Alternately, the composite particles can be separated from the aqueous medium, for instance, by freeze-drying or by spray-drying, and utilized subsequently in formulating aqueous, non-aqueous or non-liquid composition."
Unlike Alessandroni, who relied on a direct flocculation of composite slurries, Fadner employs compounding of the particulates present in composite (multicomponent) slurries with the above-mentioned coupling agents. As far as the mechanism of this compounding, or the action of the coupling agent, is concerned, Fadner explicitly states that "The scientific factors governing the formation of composite particles according to this invention are not completely understood." Moreover, Fadner does not provide any information with regard to the mechanical integrity of the dried composite particles, but an analysis of the functional aspects of the composite pigment systems in question clearly points to the lack of any practically significant adhesion mechanism capable of providing such an integrity.
Another approach to synthesizing composite pigments from blends of TiO.sub.2 and a coarse calcined clay, or a coarse delaminated clay, was disclosed in U.S. Pat. No. 3,726,700 to Wildt. It should be pointed out that although a flocculation of the particulates present in the reaction medium is unavoidable under the reaction conditions described in the above-mentioned patent, an intentional flocculation was not specified as an element of the strategy of the approach elected by Wildt. Instead, Wildt's approach was based on compounding the pigmentary raw materials present in the furnish with in-situ formed alumino-silicate or similar gels, of the type used routinely in the TiO.sub.2 industry for the application of surface coatings to TiO.sub.2 particles.
The mechanical integrity of Wildt's composite pigments was provided by thermally curing the in-situ-formed gels, called in the above patent ". . . hydrous oxide of aluminum, silicon, titanium, and mixtures thereof."
In analyzing the colloidal and kinetic aspects of Wildt's process, it is readily understood by those skilled in the art that a detrimental fractionation and selective flocculation of the pigmentary components employed, both according to species and size, are impossible to avoid in the lengthy synthesis process in which just a single step of digestion takes from 30 to 60 minutes. Furthermore, the above fractionation and selective flocculation were undoubtedly facilitated even more through the use of the dispersion-destabilizing alum. Although a permanent immobilization of TiO.sub.2 particles relative to the extender particles was indeed attained in Wildt's composite pigments, there is also virtually no doubt that the latter immobilization was realized through the attachment of "blobs" of badly flocculated TiO.sub.2 particles to the coarse extender (carrier) particles.
Perhaps the most fundamental point to be raised with regard to Wildt's composite pigments is that, in accordance with the relative proportions of TiO.sub.2 and coarse-particle-size extenders employed, the number of available extender particles was far too low to approach even remotely an effective interspacing of TiO.sub.2 particles present in the system, the oversized dimensions of the extender particles notwithstanding.
Since Wildt does not provide any numerical light-scattering data for the composite pigments in question, e.g., in a head-to-head comparison with rutile pigment used as the raw material for their synthesis, no unambiguous conclusion can be reached as to the true source of the claimed improvement of the hiding efficacy of paint systems formulated with the aid of the composite pigments in question. As is well known to those skilled in the art, however, an increased hiding efficacy of TiO.sub.2 -based paints can also be obtained by a simple addition of extraneous loose particles of high-oil-absorption silicates, of the same type as were synthesized in situ in Wildt's composite pigment.
In summary, the principal goal of keeping TiO.sub.2 particles apart by interspacing them with particles of secondary pigments (extenders) and immobilizing both species relative to each other was approached in the prior art by synthesizing both intrinsic as well as derivative TiO.sub.2 -based composite pigments, further referred to also as "extended-TiO.sub.2 " composite pigments, with the aid of three fundamentally different approaches presented schematically in the following:
(1) In-situ coprecipitation of both titanium dioxide as well as extender particles in an aqueous medium, or precipitation of titanium dioxide particles in aqueous slurries of extraneous extender particles, the choice of extenders being limited in practice, in both of the above cases, to essentially only barium sulfate or calcium sulfate. PA1 (2) Coflocculation of extraneously prepared titanium dioxide and extender pigments present in composite (multicomponent) aqueous slurries, using either extraneous flocculants or an in-situ interaction between two antagonistic (incompatible) dispersing agents in accordance with U.S. Pat. Nos. 2,176,875, 2,176,876 and 2,176,877 to Alessandroni. PA1 (3) Compounding composite aqueous slurries of extraneously prepared titanium dioxide and extender pigments with coupling agents (aliphatic acids), in accordance with U.S. Pat. No. 3,453,131 to Fadner, or with (in-situ-formed) gel-like precipitates of hydrous oxides of silicon, aluminum and titanium, in accordance with U.S. Pat. No. 3,726,700 to Wildt. PA1 (a) A maximum homooenization of the composite, well-dispersed pigment furnishes by inducing a statistically uniform spatial distribution of all particulates present in the aqueous reaction medium with the aid of suitable (high-performance) dispersants and intensive agitation regimes. PA1 (b) Instantaneous immobilization ("freezing") of the dynamically induced and maintained, statistically uniform spatial distribution of all particulates present in the furnish to permanently preserve an equivalent statistically uniform distribution of these particulates in the resultant composite pigments. PA1 (c) Intrinsic cementation of the resultant composite-pigment aggregates to impart adequate mechanical integrity to the composite pigment products, enabling them to withstand the shearing regimes to which they are routinely exposed in handling and end-use applications. PA1 (a) from 0.5% to 95%, by weight, of at least one mineral particulate subpigmentary material; PA1 (b) from 0.5% to 25%, by weight, of at least one high-oil-absorption particulate material with a specific surface area larger than 100 m.sup.2 /g; PA1 (c) from 0.25% to 20%, by weight, of at least one organic, particulate non-film-forming material; PA1 (d) from 0.25% to 5%, by weight, of at least one disperse and/or soluble organic polymer adhesive; PA1 (e) from 0.001% to 0.5%, by weight, of at least one organic, cationically active compound with at least two reactive groups in each molecule; PA1 (f) from 0.005% to 5%, by weight, of at least one color dye; PA1 (g) from 0.005% to 0.2%, by weight, of carbon black; and PA1 (h) from 0.1% to 2%, by weight, of synthetic and/or cellulosic microfibrils .
Regardless of the type of the approach employed, the three key processing elements which are indispensable to a successful synthesis of derivative composite pigments of an extended-TiO.sub.2 type are as follows:
As is readily understood by those skilled in the art, the in-situ coprecipitated, coalesced intrinsic composite pigments (TiO.sub.2 /BaSO.sub.4 and TiO.sub.2 /CaSO.sub.4) of the prior art conform to all of the above-listed requirements. For example, the requirements specified under (a) and (b) were satisfied by virtue of the intrinsic coprecipitation, providing a perfect homogenization and mutual interspacing of the primary and secondary particulate species. The requirement specified under (c), on the other hand, was satisfied by sintering the intrinsically interspaced particulate species.
Although the coalesced, extended-TiO.sub.2 composite pigments were manufactured for many years (until about 1970), their commercial demise was brought about by the inherently limited selection of viable extenders, narrowed down essentially to the unattractive barium sulfate and calcium sulfate.
Neither of the above-mentioned approaches satisfies the requirements (a) or (b) when applied to the synthesis of derivative TiO.sub.2 -based composite pigments of the prior art, as will become clear from the considerations to follow, and only Wildt's approach provides an adequate mechanical integrity to the resultant composite pigment products in accordance with the requirement (c).
First of all, a truly optimized pigment dispersion, such as is indispensable for a successful manufacture of derivative composite pigments in accordance with the requirement (a), mentioned above, can only be attained with a single monodisperse particulate species. In a slurry of a polydisperse pigment, for example, one can distinguish three distinct classes of particulates, namely, pigment fines, with particles smaller than 0.15-0.2 .mu.m e.s.d.; intermediate fractions, with particles ranging from about 0.2 .mu.m to about 0.5-0.7 .mu.m e.s.d.; and "coarse" fractions, with particles larger than 0.7-1 .mu.m e.s.d., differing significantly with regard to their behavior in aqueous media. Hence, while commercial slurries of polydisperse pigments of the above type are often referred to in the art as being "optimally" dispersed, in reality, each individual particle-size fraction is characterized by a different dispersion stability and, conversely, different resistance to flocculation.
The overall picture becomes yet more complicated with pigment slurries that are both polydisperse and heterodisperse (consisting of two or more different pigment species), such as are used for the synthesis of derivative composite pigments. It is thus impossible, for all practical purposes, to obtain an optimized dispersion of polydisperse composite slurries, in that each of the above-mentioned fractions and species has a different optimum dispersant demand, both quantitatively and qualitatively, and a different equilibrium between the dispersant adsorbed on the pigment and dissolved in the carrier medium (water). Hence, even if "optimally" dispersed slurries of individual pigments were prepared separately and then blended, a progressive destabilization of the disperse state would immediately set in. As a consequence, the polydisperse and heterodisperse phases present in the system (pigment furnish) would commence to separate (fractionate) and selectively flocculate according to species and size, starting with the relatively least stable disperse fraction and progressing toward the relatively more stable ones.
Ironically, the better the initial dispersion of a composite pigment slurry, the more pronounced are the phenomena of fractionation and selective flocculation when the slow and inefficient flocculation processes of the prior art are employed in making composite pigments.
It is thus obvious that an instantaneous immobilization of all particulate components of both heterodisperse and polydisperse pigment furnishes is indispensable for synthesizing TiO.sub.2 -based composite pigments in which the individual particles of TiO.sub.2 are either intimately interspaced with extender particles, if the latter are present in sufficient numbers and have proper dimensions, or are at least distributed in a statistically uniform fashion within the flocculated composite pigment furnish and the resultant pigment aggregates. It is also obvious, however, that the abovementioned instantaneous immobilization can only be achieved with the aid of a truly instantaneous, indiscriminate and complete flocculation mechanism.
A flocculation mechanism having the latter attributes was unknown in the prior, art, however, before it was disclosed in U.S. Pat. No. 5,116,418 to Kaliski ("Process for Making Structural Aggregate Pigments," and the co-pending patent application Ser. No. 07/919,831 ("Functional Complex Microgels with Rapid Formation Kinetics"), Filed Jul. 27, 1992, both above applications being incorporated herein by reference. According to the above disclosures, composite pigments characterized by a statistically uniform distribution of all particulate components can be synthesized only with the aid of both well-dispersed pigment furnishes and an instantaneous, indiscriminate and complete flocculation process.
It is worth pointing out, though, that while an optimized pigment dispersion, attainable only with the use of highly efficient modern dispersants, is necessary for a successful synthesis of derivative composite pigments, the weak and slow flocculating processes of the prior art are incapable of effectively overriding the powerful action of the latter dispersants. It is not at all surprising, therefore, that Alessandroni makes no references with regard to any dispersion optimization of the composite raw material slurries to be flocculated, the dispersants listed being very inefficient or outright mediocre. Fadner, on the other hand, explicitly concedes in U.S. Pat. No. 3,453,131 (col. 5, lines 1-5): "The dispersant for the functional colloidal particles should be present in minimal amounts and should not be of a type that is so strongly adsorbed to the functional particles that it cannot be replaced by, or its function overcome by, the organic acid coupling agent."
The process for synthesizing TiO.sub.2 -containing composite pigments according to U.S. Pat. No. 3,726,700 to Wildt does not make any use of dispersants, as is clear from the following description:
"1. Titanium dioxide cooler discharge and the selected clay in the desired proportions are slurried in water (the term "slurrying in water" implies in the trade that no dispersants are being used - applicant) at 100-300 g/l of total solids.
2. While vigorously agitating the slurry, a solution of sodium silicate (400 g/l SiO.sub.2) is added to the slurry in an amount necessary to give the desired weight percent of SiO.sub.2, based on total pigment solids.
3. A solution of aluminum sulfate (100 g/l Al.sub.2 O.sub.3)) is added in sufficient quantity to give the desired proportion of Al.sub.2 O.sub.3.
4. The slurry is, optionally, then heated to 50.degree.-70.degree. C.
5. Since the pH is usually less than 5 at this point, sodium hydroxide is added to the agitated slurry until the pH reaches 7.8-8.3.
6. After a digestion for 30-60 minutes, with occasional additions for pH adjustments, the slurry is filtered, washed, dried and fluid energy milled."
As is well known to those skilled in the art, it is impossible to obtain a satisfactory deaggregation, let alone dispersion, of the composite pigment furnish in the above step (1) by simply slurrying the pigmentary components in water at 10-30% solids. As a matter of fact, a satisfactory deaggregation and dispersion of a pigment, or blend of pigments, cannot be attained in a diluted slurry even with the aid of the best dispersants available.
Although sodium silicate used in step (2) has some moderate dispersing action when used in concentrated pigment slurries at very low dosages (a few tenths of one percent on the weight of pigment), it acts as a flocculant, specifically, micro-flocculant, at the high proportions specified in Wildt's patent. The above flocculating action is still further compounded by the addition of aluminum sulfate in step (3), the optional heating in step (4), and the lengthy digestion of 30-60 minutes in step (6).
It is thus obvious that the principal goal of the above-discussed synthesis procedure, clearly defined by the following quotation from U.S. Pat. No. 3,726,700 to Wildt (col. 2, ln. 5-9), has not been attained: "It would be advantageous, therefore, if the titanium dioxide could be affixed to the inexpensive extender in such a manner that the optimum spacing could be maintained throughout the various processes of paint preparation, pigment dispersion and drying."
Wildt also states (col. 2, lines 13-19) that "A process for affixing pigment particles of colloidal size to particles of a non-swelling clay by coupling with organic acids is described in Fadner U.S. Pat. No. 3,453,131 but there is no suggestion that in using TiO.sub.2 as the pigment it is possible to achieve a composite pigment of outstanding superiority in terms of light scattering efficiency."
While the above statement regarding the context of Fadner's disclosure is basically correct, Wildt himself has not offered any direct evidence that his composite pigments do indeed provide such a superior light-scattering efficiency. As previously indicated, the increased hiding in paints, claimed by Wildt, can also be obtained by indirect means without increasing the intrinsic light-scattering efficacy of the composite pigment itself. As a matter of fact, neither Wildt, nor Fadner, nor Alessandroni have offered any objective evidence that the immobilized TiO.sub.2 particles in the composite pigments in question have indeed acquired steric configurations favoring increased light scattering, even though such an evidence can be secured readily in terms of light-scattering data or electronphotomicrographs.
In an analogy to the path of reasoning pursued by Fadner, Wildt postulates as follows: "That the product thus obtained consists of particles of TiO.sub.2 which are dispersed on, and adhered to, the larger clay particles is shown by the lack of selective sedimentation when dispersed in water, the unchanged composition after fluid energy milling, and the markedly improved hiding power conferred to latex paints when compared to a mechanical mixture of equivalent TiO.sub.2 -clay composition."
As is readily understood by those skilled in the art, however, the above "proofs" constitute conditions which are merely necessary, but not sufficient, to reach the above conclusion in that (a) the thermally cured alumino-silicate cement formed in situ in Wildt's process is sufficiently strong to permanently affix TiO.sub.2 and extender particles (regardless of whether their steric configurations are overall favorable or not), to prevent any subsequent separation in handling and end-use applications; and (b) a permanent, durable, cementation of the individual particulate ingredients of composite pigments does not prove that affixing TiO.sub.2 particles ". . . to the inexpensive extender in such a manner that the optimum spacing could be maintained throughout the various processes . . . ," postulated in the patent in question, has been even remotely approached.
The issue of extending TiO.sub.2 pigments, treated extensively in the literature in the last several decades, has been most fittingly summarized by J. H. Brown in the article titled "Crowding and Spacing of Titanium Dioxide Pigments," issued in the Journal of Coating Technology, Vol. 60, No. 758, Pages 67-71March 1988, dealing with hiding properties of nonporous paints. In the above article Brown dismisses the usefulness of particulate extenders, opting, instead, for coatings deposited on the surface of TiO.sub.2 particles. Brown finally concludes that, under the best of circumstances, the hiding power of coated TiO.sub.2 pigments in paints with a pigment-volume concentration greater than 20% could be as much as 10% higher than that of a conventional (uncoated) rutile, which is not a major improvement by any means.
While the industrial experience with derivative composite pigments of the extended-TiO.sub.2 type, obtained by co-flocculation or compounding with extraneous or in-situ synthesized agents, has been consistently negative, it should be emphasized that the principal reason for the above lack of a practically acknowledged success was that the approaches known in the prior art, based on slow and inefficient flocculation processes, were incapable of preventing the detrimental segregation and selective flocculation of pigmentary components during the course of the synthesis process.
It is also worth noting that while all composite pigments are, de facto, aggregates, the aggregation as such, specifically, a controlled aggregation, was never intentionally employed in the prior art as an independent tool for enhancing the optical properties of composite pigments. Instead, the latter enhancement has always been attempted through interspacing of TiO.sub.2 particles with particles of extender pigments, steadfastly ignoring the fact that pigments (extenders) with particle dimensions small enough to be suitable for a truly effective interspacing have never been available on the market. It must thus be stated unequivocally that, with the exception of the intrinsic, coprecipitated TiO.sub.2 /BaSO.sub.4 (CaSO.sub.4) composite pigments, mentioned previously, the prior-art references relating to interspacing of TiO.sub.2 particles with extender particles are a clear evidence of the prevailing confusion with regard to the above subject matter.
The fact that light-scattering properties of entire pigment populations can be improved by aggregating in situ pigment fines, whose dimensions in a discrete state are too small for efficient light scattering, was first discovered by the applicant and published in the Journal of the Technical Association of the Pulp and Paper Industry (TAPPI), Vol. 53, No. 11, November 1970, Pages 2077-2084 ("Performance of Some Clays in Starch-Containing Paper-Coating Films; Part I. Black Glass Plates as Model Substrates"), preceded by a presentation at the TAPPI Coating Conference held in Houston, Tex., May 3-4, 1970.
The above publication shows plots of light-scattering coefficients at the wavelengths of 457 and 577 nm as a function of the binder-volume fraction (FIGS. 6 and 7), assessed for three different clay pigments made into coating colors and deposited as films on optically flat black glass plates as coating substrates. The slopes of the curves representing the light-scattering coefficients of No. 1 and No. 2 clays ascend initially with the increasing binder-volume fractions and, after reaching the maximum values at a binder-volume fraction corresponding to about 5 parts of starch per 100 parts of clay, by weight, descend as the binder level is further increased.
This initial increase of the light-scattering coefficients is explained in the above publication ". . . by an aggregation of clay fines effected by the initial addition of binder. The aggregates of ultrafine particles, which are understood here as assemblies of a very few such particles, should scatter the light more effectively than the individual components." The subsequent steady decline of the magnitude of the light-scattering coefficients is explained as follows: "An increase of the binder content of the coating systems beyond the F.sub.bv (binder-volume fraction-explanation added by the applicant) value of 0.080 (5 parts starch per 100 parts clay, by weight) appears to cause a further growth of the assemblies of pigment particles, so that the optimum dimensions of the light-scattering sites are exceeded."
With the relatively coarse mechanically delaminated clay, having a low proportion of particles with equivalent spherical diameters in the 0.1 .mu.m to 0.2 .mu.m range, the light-scattering coefficients of the coatings declined from the very first incremental addition of binder because of the scarcity of ultrafine particles amenable to a beneficial aggregation. The relatively coarser intrinsic structure of coating films containing the mechanically delaminated clay, compared to those containing No. 1 and No. 2 clays, has been verified with the aid of the empirical parameter called "Rho" (after the Greek letter .rho.), defined in the above publication as the ratio of light-scattering coefficients determined at 577 nm and 457 nm for the same coating film.
With coating films characterized by intrinsic structures that are relatively fine, such as binderless coatings or coatings having a low binder-volume fraction, the magnitudes of the corresponding "Rho" parameters are low. As the intrinsic coating structures become coarser, as was the case with all coatings discussed in the above publication in which the binder content was increased, the magnitude of Rho increases accordingly.
It becomes apparent on the basis of the above considerations, backed by practical experience, that the light-scattering efficacy of end-use formations containing pigments depends not only upon the original shapes and dimensions of pigment particles but, to a major extent, also on how these particles aggregate (flocculate) into the resultant end-use formations. The publication by Kaliski (TAPPI Journal, Vol. 53, No. 11, November 1970, pages 2077-2084) thus established scientific foundations for a new pigment technology opening the way to designing and manufacturing entirely new lines of derivative pigment products with improved optical and functional performance properties, synthesized by a controlled aggregation of extraneous pigmentary raw materials by themselves or combined with a variety of subpigmentary and/or nonpigmentary particulates.
Indeed, the first patent pertaining to the manufacture of aggregate pigments with an improved optical performance (U.S. Pat. No. 4,075,030: High Bulking Clay Pigments and Methods for Making the Same) was issued in 1978 to Bundy et al., with related patents by other inventors to follow. It should be pointed out, though, that none of the patented aggregate pigments was synthesized under conditions allowing a satisfactory control of the aggregation process, since the later control can only be achieved with the aid of an instantaneous, indiscriminate and complete flocculation. A flocculation process of the above-mentioned type was unknown in the prior art, however, before the previously mentioned disclosure by the applicant. Moreover, as documented amply by industrial experience, the problem of imparting an adequate mechanical integrity to aggregate pigments, while simultaneously generating in a controlled manner beneficial aggregate structures, has never been resolved satisfactorily in the technology of aggregate pigment products of the prior art.
Novel methods for the manufacture of practically countless types of aggregate pigments with exotic compositions, enhanced optical properties, excellent mechanical integrity, and unique functional properties, have been disclosed in U.S. Pat. No. 5,116,418.
It is intended hereinafter, therefore, to demonstrate how novel derivative composite pigment products can be synthesized by a controlled aggregation of primary and secondary (extender) pigments dispersed in aqueous media (furnishes), optionally including various subpigmentary and nonpigmentary particulates and auxiliary soluble media. It is also intended to provide an unambiguous classification of derivative aggregate composite pigment products of the extended-TiO.sub.2 type to clearly distinguish between those intrinsically interspaced ones and those of a "pseudo-interspaced" type characterized merely by a statistically uniform distribution of the particulate components.
In accordance with the foregoing and disclosures to follow, it is an object of the present invention to provide formulations for novel composite pigment products containing from 4.5% up to 50%, by weight, of particulate titanium dioxide, synthesized by the general method disclosed in the previously mentioned U.S. Pat. No. 5,116,418, the optical-performance efficacy of the composite pigments under discussion being superior to that of the equivalent loose blends of titanium dioxide and raw materials employed for their synthesis.
In particular, it is an object of the invention to provide formulations for composite pigment products containing from 4.5% up to 50%, by weight, of particulate titanium dioxide, encompassing both rutile and the slightly less efficient anatase, derived from commercial titanium dioxide pigment products in the state "as is," or comminuted beyond the limits of comminution practiced in the prior art.
It is also an object of the invention to provide formulations for composite pigment products containing from 4.5% up to 50%, by weight, of particulate titanium dioxide combined with extraneously prepared, low-refractive-index inorganic and/or organic pigments.
It is a further object of the invention to provide formulations for composite pigment products containing from 4.5% up to 50%, by weight, of particulate titanium dioxide, also containing extraneously prepared particulate materials which are subpigmentary and/or nonpigmentary.
It is a yet further object of the invention to provide formulations for composite pigment products containing from 4.5% up to 50%, by weight, of particulate titanium dioxide, also containing carbon black to impart extra-high opacifying properties to the latter pigment products, thus rendering them especially well suited for applications such as the manufacture of lightweight newsprint or paints and lacquers with superior hiding properties.
It is a still further object of the invention to provide formulations for composite pigment products containing from 4.5% up to 50%, by weight, of particulate titanium dioxide, also containing from 0.005% to 5%, by weight, of color dyes to eliminate the residual yellow hue inherent to all commercial TiO.sub.2 pigments and/or render the resultant composite pigments directly applicable to coloring of paints, plastics and synthetic fibers.
It is a yet further object of the invention to provide formulations for composite pigment products containing from 4.5% up to 50%, by weight, of particulate titanium dioxide, also containing from 0.001% to 0.5%, by weight, of chemically built-in organic, cationically active compounds with at least two reactive groups in each molecule to impart controlled levels of organophilic properties to the resultant pigment products, thus rendering them uniquely compatible with, and dispersible in, organic media such as plastics, synthetic fibers and solvent-based lacquers and paints.
It is a yet further object of the invention to provide formulations for composite pigment products containing, in addition to extender pigments and other auxiliary materials, from 4.5% up to 50%, by weight, of particulate titanium dioxide, in which the particulate ingredients are coflocculated into aggregates whose intrinsic structure and spatial distribution of the light-scattering sites and functional sites provide optical- and functional-performance efficacies that are superior to those of the equivalent loose blends of TiO.sub.2 and other raw materials employed.
It is also a particularly special object of the invention to provide general principles of qualitative, quantitative and functional formulating to enable one to custom design composite pigment products containing from 4.5% up to 50%, by weight, of particulate titanium dioxide in combinations with other particulate components, having overall optical and functional-performance characteristics superior to those attainable with the aid of the equivalent blends of loose (nonaggregated) particulate raw materials.